Known sterilization methods are autoclaving, in other words the use of moist heat, irradiation with ionizing rays, gas sterilization with ethylene oxide (ETO), plasma sterilization and hydrogen peroxide sterilization.
Autoclaving achieves a very good sterilization effect but cannot be used for thermally labile items.
Ethylene oxide (ETO) is a highly toxic gas used for sterilization of thermally labile materials, but it has the disadvantage of relatively long outgassing times of the items to be sterilized, since these outgassing times may be several times longer than the actual treatment time. Because of the toxicity, proper handling of ETO is critical.
Disadvantages of hydrogen peroxide sterilization can be seen primarily in the fact that hydrogen peroxide is a liquid at room temperature. The known methods are based on evaporation of the hydrogen peroxide, but then condensation in the region of the objects to be sterilized must be avoided to ensure that the vapor phase can be extracted completely. The removal of hydrogen peroxide takes place only poorly if large amounts of liquid have accumulated. In this case evaporation is irregular and lasts a long time.
The treatment with ionizing radiation imposes major requirements on equipment and safety. Furthermore, in some cases ionizing radiation may damage the material of the objects being sterilized. The known plasma-sterilization methods do not suffer from these disadvantages or do so to only a limited extent. The sterilizing effect of a plasma depends among other factors on destruction of the germs and biomolecules by the combination of UV and VUV radiation, which destroys the DNA. One disadvantage is that sterilization of heat-sensitive items is usually possible only with increasingly negative pressure.
It has been shown, however, that plasma methods known heretofore have only limited suitability for the treatment or decontamination and sterilization of complex structures, since, for example, the ability of the plasma to penetrate into narrow gaps and lumens is not very good, even under high vacuum.
Another problem is the treatment with sterilizing gases and substances (such as ethylene oxide and hydrogen peroxide), which in higher concentrations are sometimes highly explosive. These methods also require lengthy post-treatment phases to eliminate residues of the highly toxic substances. Thus the storage and handling of these substances are complicated and expensive for the user.
US 2008/0317626 A1 describes a method and a device in which a compound (carbon-based diazenium diolate compound and a powdered acid) that generates a sterile gas is used, in this case to generate preferably NO or a mixture of NO and NO2 as the sterile gas.
US 2010/0166603 A1 describes a sterilization method for powder, using NO2 and moisture as the sterilizing gas. Liquid NO2 is used as the source for gaseous NO2.
A disadvantage of this method is that here also chemical substances are stored and used for formation of the necessary active gases. The times of action necessary are one hour and longer.
WO 2010/022871 describes a non-thermal plasma-sterilization method that describes air among other substances as the carrier gas and uses various additives. NOx and H2O2, for example, are mentioned as active species. The use of a non-thermal plasma indeed permits in principle the generation of species with antimicrobial activity and thus also killing kinetics to the extent of several log steps, but a sterilizing effect for the most important reference germs and spores cannot be achieved by the small production of the germ-killing species. Safe and reproducible sterilization, especially even in cases of larger throughputs of items to be sterilized, cannot be achieved in practice with this method.
A known phenomenon is the sterilizing effect of plasma-generated gas, as is also described in US Patent Application 2010/0254853 A1. The effect of plasma-generated gas depends mainly on the generation of NOx and free radicals, with the associated long times of action of one hour and longer.